Cortical responses to promontorial stimulation in postlingual deafness

Hearing Research 209 (2005) 32–41 www.elsevier.com/locate/heares

Cortical responses to promontorial stimulation in postlingual deafness Malene Vejby Mortensen

a,b,c,¤

, Stig Madsen b, Albert Gjedde

a,c

a

b

PET Center, Aarhus University Hospitals, 44 Norrebrogade, 8000 Aarhus, Denmark ENT Department, Aarhus University Hospitals, 44 Norrebrogade, 8000 Aarhus, Denmark c Center of Functionally Integrative Neuroscience, Aarhus University, Aarhus, Denmark Received 6 May 2005; accepted 12 May 2005 Available online 11 August 2005

Abstract Electrical stimulation with a transtympanic electrode on the promontory of the middle ear allows the tasks of gap detection and temporal diVerence limen (TDL) to be carried out by both normally hearing and deaf subjects. Previous neuroimaging of normally hearing subjects revealed a region in the right posterior temporal lobe that is crucial to duration discrimination. The present study tested the hypothesis that postlingually deaf subjects recruit this area when they make subtle temporal discriminations. Fourteen postlingually deaf adult cochlear implant candidates were stimulated in the ear chosen for implantation. Altered cerebral activity was recorded with positron emission tomography as incremental 15-O-labelled water uptake. On stimulation with tone bursts, we found bilateral activity close to the primary auditory cortex in all subjects. However, subjects performing well on the TDL task demonstrated right-lateralized fronto-temporal and left-lateralized temporal activity in the respective TDL and gap-detection tasks, while subjects who failed to detect duration diVerences of less than 200 ms in the TDL discrimination task only had frontal and occipital rather than temporal lobe activation. We conclude that the ability to involve the right posterior temporal region is important to duration discrimination. This ability can be evaluated pre-operatively.  2005 Elsevier B.V. All rights reserved. Keywords: Cochlear Implant; PET; Promontorial test; Temporal processing

1. Introduction Promontory testing is a crude electrical stimulation of the cochlea that pre-surgically evaluates the function of the auditory nerve and the temporal processing ability of cochlear implant candidates. The Nucleus Promontory Stimulator unit allows for subjective assessment of gap Abbreviations: BA, Brodmann area; CI, cochlear implant; rCBF, regional cerebral blood Xow; DR, dynamic range; GAP, gap detection; MAL, maximum acceptable loudness; MCL, most comfortable level; MR, magnetic resonance; PET, positron emission tomography; PTA, pure tone average; TDL, temporal diVerence limen; TL, threshold level * Corresponding author. Tel.: +45 8949 3081/4408; fax: +45 8949 4400. E-mail addresses: [email protected], [email protected] (M.V. Mortensen). 0378-5955/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.heares.2005.05.011

detection and duration discrimination (temporal diVerence limen, TDL) abilities, tasks which require temporal processing (Eddins and Green, 1995). Correlations between the results of pre-operative electrical stimulation and post-operative performance vary in previous studies. Of the two tests, the TDL has been demonstrated to be of greater predictive value (Black et al., 1987; Waltzman et al., 1990; Blamey et al., 1992; van Dijk et al., 1999). In normal hearing, speech processing predominantly, but not completely, is left lateralized (Binder et al., 1997; Gjedde, 1999; Springer et al., 1999). In contrast, nonspeech processing is known to be more right lateralized when the stimulus is tonal (Binder et al., 2000; Zatorre et al., 2002), whereas non-speech, in the form of pseudowords or time-reversed sentences, is more bilaterally processed (Binder et al., 2000; Wong et al., 2002). A

M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

consistent Wnding in cochlear implant users listening to speech is the more extensive right temporal activity, i.e., the presence of more bilateral activity than characteristic of language processing in normal hearing (Wong et al., 1999; Giraud et al., 2000, 2001). The reason for this diVerence is not known. Tones commonly are used to study temporal processing, as processing of speech signals is held to be more complex and to involve analysis of other properties of speech. Imaging of this processing has yielded discrepant interpretations, however, including claims of both rightlateralized (GriYths et al., 1999; Rao et al., 2001) and left-lateralized (Robin et al., 1990; Zatorre and Belin, 2001) activities. Both posterior temporal lobes are active during the temporal analysis of sounds in general (GriYths et al., 1998), but the right posterior temporal lobe speciWcally is involved in duration discrimination tasks involving both auditory and tactile stimuli (Pardo et al., 1991; Belin et al., 2002). Temporal coding is suYcient for speech comprehension (Shannon et al., 1995; Moller, 1999). Perception of phonemes primarily depends on the relative timing of acoustic events within the range of temporal diVerences of the order of tens of milliseconds (Ladefoged, 2000). Also, in some early studies, cochlear implantees showed a correlation between temporal resolution and the perception of consonants (Cazals et al., 1991; Muchnik et al., 1994). Recent animal studies suggest that plastic changes in temporal processing of central auditory neurons contribute to the variability of the outcome and the gradual improvement in speech recognition after implantation (Vollmer et al., 1999). Thus, we reason that knowledge of the ability to process temporal information before cochlear implantation is important to the evaluation of the outcome. In a previous study, we combined promontory electrical stimulation with functional brain imaging by PET in normally hearing subjects (Mortensen et al., 2005). The test was chosen to assess brain function of normally hearing individuals during exposure to non-verbal temporal information presented in a manner similar to the stimulation applied to cochlear implant users. In that study we demonstrated that a right-lateralized frontotemporal network of active neurons provides the neuronal basis for duration discrimination, irrespective of the subjective nature of the sensation, be it auditory or somatosensory. This study, as well as an earlier study showing the same right posterior temporal region to be involved in phoneme discrimination (Pedersen et al., 2000), led us to conclude that this region could be a major neuronal substrate of phoneme identiWcation, a task which requires analysis of subtle temporal diVerences. We further suggested that gap detection is of little value in the assessment of cortical integration and rather appears to engage neuronal competence at a subcortical level (Mortensen et al., 2005).

33

On the basis of these results, we formulated the hypothesis that activity in the posterior right middle temporal gyrus and the right prefrontal cortex distinguish postlingually deaf subjects with the ability to discriminate smaller duration diVerences between two successive stimuli (100 ms or less) from subjects who need larger diVerences (more than 100 ms) to make the same judgment. We used positron emission tomography of promontory stimulation to test the hypothesis in candidates of cochlear implantation. We also tested whether gap detection provided additional information about cortical processing of temporal information in postlingually deaf subjects.

2. Materials and methods 2.1. Subjects Fourteen postlingually deaf adult CI-candidates were included in successive order after recommendation for cochlear implantation. They were stimulated in the ear chosen for implantation. The subjects formed two groups on the basis of the results of pre-operative promontory electrical stimulation. One group of six subjects (Wve men, one woman, with mean age of 53.5 years) had excellent sound duration discrimination (temporal diVerence limen, TDL 6 100 ms) while another group of eight subjects (three men, Wve women with mean age of 46.7 years) had poor sound duration discrimination (TDL 7 200 ms). Two subjects in the highly discriminating group and three in the poorly discriminating group were stimulated on the left side. Generally the right ear is chosen for implantation (considered practical for right-handed candidates) and favorable to the right ear/left hemisphere dominance for speech sounds revealed by dichotic listening studies (Best and Avery, 1999), unless speciWc pre-operative information favors the left ear. The most common Wnding is preservation of minor residual hearing for low frequencies, as was the case in three of the Wve patients. In one patient the right cochlea was ossiWed and in another gap detection and TDL on promontory testing were demonstrated only on the left side. The Wrst promontory test was made before inclusion in the study and hence subjects were familiar with the procedure when the test was repeated immediately prior to the scanning session by advance of a needle through the tympanic membrane to the promontory of the middle ear. The second test was made immediately prior to scanning to determine the most comfortable level (used for stimulation during PET) and to conWrm the thresholds for gap detection and TDL. The promontory test was previously described in detail (Mortensen et al., 2005) and the pre-scan data are listed in Table 1.

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M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

Table 1 Clinical data Patient

Sex

Handedness

Age

Duration of profound hearing loss

Aetiology of hearing loss

Side of stim.

MAL

TL

DR

Gap

TDL

PTA

A B C D E F 1 2 3 4 5 6 7 8

M M M M F M F F M M F F F M

L/R R R/L R R R R R R R R R R R

54.7 64.4 30.8 56.3 43.5 71.4 47.5 32.6 52.3 51.3 51.4 50.2 38.8 49.4

1 5 10 20 1 4 15 32 1 40 20 25 33 10

Prog. after meningitisa Mb.Meniere and trauma Trauma PSNHLb Cogan’s syndrome Otosclerosis PSNHL Streptomycin PSNHL PSNHL PSNHL PSNHL Meningit PSNHL

R R R R L L R R R R R L L L

318 410 138 146 480 270 268 127 680 130 200 300 350 197

177 180 60 104 82 140 180 73 320 52 70 150 230 82

5.1 7.2 7.2 2.9 15.3 5.7 3.5 4.8 6.5 8 9.1 6 3.6 7.6

150 50 150 100 50 200 >250 100 150 >250 150 250 >250 >250

50 100 70 10 50 100 >250 200 200 >250 >250 200 >250 200

110 110 >120 110 >120 >120 100 >120 >120 115 100 >120 >120 100

MAL, maximum acceptable level; TL, threshold level; DR, dynamic range; PTA, pure tone average. Pre-operative lip reading score in an open set standard test. a Meningitis in childhood. b Progressive sensorineural hearing loss.

BrieXy, the promontory stimulator (Nucleus®, Model Z10012, Cochlear Limited) delivers an electrically isolated constant-current square wave stimulus. The patients were scanned during three conditions of electric stimulation at the frequency of 100 Hz including tone bursts of 500 ms every second, a gap-detection test and a temporal diVerence limen test (TDL), which is a test of duration discrimination. The tasks were performed silently to avoid possible contamination of the activation pattern by motor-speciWc activity.

2.3. Data analysis We used a Wxed-eVects model with condition, subject, and global eVects, and appropriate contrasts to create statistical parametric maps of the t-statistics. The main contrasts were tone burst against rest, and the gap-detection task and temporal diVerence limen task against rest. Data were analyzed for each patient individually by computing the diVerence in image between stimulation and rest conditions.

2.2. PET image acquisition and processing We measured raised or reduced cerebral activity as the change of the brain uptake of oxygen-15-labeled water, which matches the distribution of cerebral blood Xow, using an ECAT Exact HR 47 PE-Tomograph (Siemens/CTI). A single 60-s frame was acquired, starting at 60,000 true counts per second after repeated i.v. bolus injections of doses of tracer with an activity of 500 MBq. The tomography took place in a darkened room with subjects’ eyes closed. A silent baseline condition was duplicated, generating a total of Wve tomography sessions. Stimulation commenced 10 s before the injection of the tracer dose. PET images were realigned using automatic image registration (AIR) software to correct for head movements. Normalization, smoothing (14 mm Gaussian Wlter) and statistical analysis were all performed with the statistical parametric mapping program DOt (Worsley et al., 1992) with signiWcance at P < 0.05 indicated by the calculated t-values, typically >4.3 in this study. The averaged MRvolumes anatomically localized the sites of increased rCBF.

3. Results In all subjects, regardless of side of stimulation and performance in the temporal tasks, the subtraction of baseline from electrical bursts resulted in a bilateral increase of activity at coordinates placed lateral and posterior to the primary auditory cortex, as anatomically deWned by Rivier and Clarke (1997) from the distribution of cytochrome c oxidase density (Table 2 and Fig. 1). A previous study of normally hearing subjects undergoing promontorial stimulation during PET scanning (Mortensen et al., 2005) revealed incremental activity in the TDL task but not in the gap-detection task. Based on this result, we focused on the diVerences between the group of subjects able to perform duration discrimination at a level close to normal hearing, and the group of subjects able to discriminate only longer durations. We found performances on the two tasks to be signiWcantly correlated (Fig. 2) and hence we analyzed the cortical processing of gap detection separately for the groups of poor and good performers of TDL.

M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

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Table 2 Regions of signiWcant increase Talairach coordinates x

y

z

¡54 59 ¡55 58 52 ¡4

¡54 ¡43 ¡36 ¡14 8 ¡85

10 10 3 0 ¡15 27

t-Value

Area

BA

6.03 5.61 4.25 4.29 5.85 7.54

Left middle temporal gyrus Right superior temporal gyrus Left superior temporal gyrus Right superior temporal gyrus Right superior temporal gyrus (anterior) Left cuneus

39 22 22 22 21/38 19

ns ns nf

ns: not signiWcant. nf: a random, not a Wxed eVect Toneburst-baseline (N D 14).

Fig. 1. Group average of 14 CI-candidates stimulated with electrical bursts through needle placed on promontory of right (nine) or left (Wve) middle ear. All subjects showed activation within primary or secondary auditory cortex. Images at left show peak foci just below signiWcance. Occipital activity was not present in all subjects.

middle temporal gyri as well as in the right anterior superior temporal gyrus (Fig. 3). In contrast, the activations of the duration discrimination task were restricted to the right posterior middle temporal gyrus and the right prefrontal cortex (middle frontal gyrus, BA9). In addition and most signiWcantly, the left cerebellum was activated when subjects performed the TDL task (Fig. 4). The coordinates are listed in Table 3. In one subject of this group, data from the gap-detection condition were not obtained. 3.2. Poor TDL performers Fig. 2. Relationship between performance in gap detection and TDL (slope signiWcantly non-zero, P D 0.012).

There was no correlation between hearing thresholds and performance of subjects in the temporal tasks. 3.1. Excellent TDL performers In the group of excellent TDL performers, gap detection raised activity in the left posterior and left anterior

Performance of the temporal tasks in this group did not raise activity in the temporal lobes. Frontal lobe activity was present in both gap detection and TDL, and bilaterally in gap detection, with activity in both the left medial frontal gyrus and the right superior frontal gyrus. Activity was seen only in the right superior frontal gyrus (BA11) in the TDL task. Gap detection raised activity also in extrastriatal cortex (Table 3 and Figs. 3 and 4).

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M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

Fig. 3. In subjects with excellent temporal skills (N D 6), gap detection raised activity mainly in left posterior middle and right anterior superior temporal gyrus (square box) whereas subjects with poor temporal skills (N D 8) only engaged frontal and occipital areas during this task.

Fig. 4. In duration discrimination, blood Xow increased in right posterior middle temporal gyrus, right prefrontal cortex and left cerebellum (square box) in subjects able to distinguish diVerences of 100 ms or less (N D 6). No signiWcant temporal lobe activity was registered in group of subjects (N D 8) with poor discriminatory ability (right image).

3.3. Deactivations Decreases were found in frontal regions in the gapdetection task and in posterior regions including occipital cortex in the TDL task (Table 4) in both groups.

4. Discussion The present study used PET to map auditory function in postlingually deaf adults. PET measurements are of two kinds, those that map a state, baseline or activation, and those that map a change or increment, caused by a perturbation of almost any kind and showing only those areas in which a signiWcant change in brain activity has occurred. The former commonly provides a measure of metabolism of glucose, determined over a period of 45 min with a labeled glucose analog (FDG), or of oxygen, determined over a period of just 3 min with

labeled oxygen, or measures of blood Xow or receptor binding, determined over 3 min with labeled water or over 90 min with a radioligand. The latter is said to provide a map of a brain function and is mostly performed with labeled water in a single minute. The former studies can have diagnostic or predictive value on the basis of their state description, while the latter can have diagnostic or predictive value on the basis of the demonstration of the functional change elicited by a speciWc physiological or pathological perturbation. Although it is questionable to which extent the promontorial stimulus yields an auditory or somatosensory sensation, or whether demonstration of a cortical response is necessary or suYcient as an evaluation of an intact auditory pathway (Silverstein et al., 1994; Truy et al., 1995; Schmidt et al., 2003; Mortensen et al., 2005), an important result of the present study is the characterization of promontorial stimulation as a test of auditory nerve function, as used in pre-operative assessment of CI

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Table 3 Regions of signiWcant increase Talairach coordinates x

t-Value

y

Area

BA

z

Excellent TDL performers <100 ms Gap-baseline ¡54 ¡55 59 ¡17

¡47 ¡18 ¡9 ¡79

6 ¡13 ¡6 ¡21

6.41 4.5 5.06 4.41

Left sup./middle temporal gyrus (posterior) Left middle temporal gyrus (anterior) Right superior temporal gyrus (anterior) Left cerebellum

ns

60 40 ¡12

¡28 10 ¡78

0 36 ¡25

4.81 4.8 5.77

TDL-baseline Right middle temporal gyrus (posterior) Right middle frontal gyrus Left cerebellum

39 21 21/22

21 9

Poor TDL performers >200 ms Gap-baseline ¡5 ¡8 28

¡76 5 60

¡8 60 ¡15

4.73 5.79 4.57

Left lingual gyrus Left medial frontal gyrus Right middle frontal gyrus

18 6 11

38

47

¡16

4.53

TDL-baseline Right middle frontal gyrus

11

ns: not signiWcant.

Table 4 Regions of signiWcant decrease Talairach coordinates x

t-Value

y

Area

BA

z

Excellent TDL performers Gap-baseline 4 27 ¡9 12

¡26 ¡13 ¡21 ¡40

65 60 59 62

¡5.51 ¡4.91 ¡4.86 ¡4.49

¡21 ¡11 ¡20 11 24 ¡4

¡33 ¡73 ¡68 ¡57 ¡33 ¡2

¡15 ¡3 8 15 ¡9 ¡14

¡5.32 ¡5.31 ¡5.08 ¡4.88 ¡4.48 ¡4.48

Right medial frontal gyrus Right precentral gyrus Left medial frontal gyrus Right postcentral gyrus

6 6 6 3

TDL-baseline Left parahippocampal gyrus Left lingual gyrus Left cuneus Right precuneus Right parahippocampal gyrus Brain stem

36 18 30 23 36

Left lateral frontoorbital gyrus

11

Left posterior cingulate Left cerebellum

23

Poor TDL performers Gap-baseline ¡8

17

¡26

¡5.07

¡1 ¡12

¡61 ¡57

14 ¡20

¡4.83 ¡4.54

TDL-baseline

candidates. The results suggest that further studies of this method in combination with brain imaging could provide important information about the neural basis on which the success of a cochlear implant depends. The results may point to new ways of reWning the treatment and they may provide a framework for future research on rehabilitation strategies that optimize outcome irrespective of pre-operative condition.

4.1. Electrical bursts All subjects classiWed the stimulus as sound. In initial analysis, we identiWed a common response to the simple stimulation with electrical bursts. This was done in order to establish comparable conditions of the two groups prior to execution of the temporal tasks. The primary auditory cortex can be deWned on functional or

38

M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

Fig. 5. Average neuroanatomical position of human primary auditory cortex (PAC) function in Talairach space as summarized by Johnsrude et al. (2002). Present sites shown in red. Approximately signiWcant (upper) sites within both hemispheres are consistent with spatial extent of PAC. SigniWcant (lower) sites of activations situated more posterolaterally may correspond to secondary rather than primary auditory cortex. (For interpretation of the references to color in this Wgure legend, the reader is referred to the web version of this article.)

neuroanatomical grounds. DeWning the average functional position of the human primary auditory cortex (PAC) in Talairach space as the signiWcantly activated regions summarized by Johnsrude et al. (2002), sites within both hemispheres (Table 2) were consistent with this spatial extent of the PAC (Fig. 5). The signiWcant activations situated more posterolaterally may correspond to secondary rather than primary auditory cortex (Fig. 1). In a recent fMRI study the secondary auditory cortex had signiWcantly more activity than the PAC (although this area was activated too) under similar stimulation conditions (Schmidt et al., 2003). Regions lateral to the PAC may mediate early analysis of stimulus complexity (Hall et al., 2002). In all subjects, activity in the auditory cortex implies functional connections in the auditory pathway from cochlea to the brain (Smith and Simmons, 1983). Activity in the occipital lobe was randomly distributed among subjects. Occipital activity was not observed in normal hearing (Mortensen et al., 2005), and hence could be the result of brain plasticity in response to years with deafness with gradual development of new processing strategies that rely on visual cues in addition to the auditory stimuli (Giraud et al., 2001). We found no correlation between lip reading ability and occipital blood Xow change in the present study. Activity did not reXect side of stimulation, in agreement with previous reports on simple auditory stimuli (Devlin et al., 2003; Schmidt et al., 2003). Metabolic responses to auditory stimuli seem to be aVected by the content of the stimulus and the processing strategy of the subject more than by the side of the stimulation (Mazziotta et al., 1982).

4.2. Temporal diVerence limen The right posterior temporal activity observed in subjects performing well on the TDL task, with peak coordinates close to those found in normal hearing (Mortensen et al., 2005), supports the hypothesis that recruitment of this region is crucial to the subjects’ ability to discriminate subtle temporal diVerences (<100 ms). Performance involved right prefrontal cortical mechanisms (middle frontal gyrus) that engage working memory during the performance of duration judgments (Mesulam, 1998; Pouthas et al., 2000). In a major study of brain lesions, speciWc areas of frontal and parietal cortex of the right hemisphere were assigned an essential role in support of time-dependent discriminations. Duration perception deWcits of the order of tens of milliseconds were associated with damage in the middle and superior frontal gyri of the right hemisphere (Harrington et al., 2004). Mazziotta et al. (1982) found the association areas in the superior and middle temporal regions to be more active on the right side during non-verbal stimulation. The posterior association area in the superior temporal gyrus extended into the inferior parietal cortex and showed the same right-side preference for non-verbal stimuli. Because poor performers succeeded on the TDL test when the diVerence was large, the results imply that the ability to discriminate Wne temporal diVerences in the range covered by acoustic speech segments (10–150 ms) is associated with the activity located in the right middle temporal gyrus in the present study.

M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

39

Both completely bilateral (GriYths et al., 1998) and more left-lateralized (Robin et al., 1990; Belin et al., 1998b; Zatorre and Belin, 2001) activities have been found in studies of temporal processing. Although the left-lateralized processing appears to be more generally associated with tasks requiring rapid temporal parsing, temporal processing has yielded divergent results also in previous studies. Thus, in a PET study of duration discrimination, Belin et al. (2002) found right temporal activity at approximately the same coordinates (66, ¡30, ¡2), but also parietal increases of blood Xow. The subjects were trained to detect sounds of a duration that deviated from a standard duration. The report did not state the actual magnitude of the deviation, but the rCBF increased with task diYculty in the right posterior middle temporal gyrus. The number of trials per scan exceeded that of the present study, suggesting a greater attention demand; this may account for the parietal activity (Belin et al., 2002). The same fronto-temporal (at peak coordinates 56, ¡34, 6) and parietal activations in concert mainly with the left cerebellum was demonstrated by the same authors in a sound intensity discrimination study, which speaks against exclusive involvement of these areas in duration discrimination. However, perception of the duration of intensity change periods relative to the standard intensity periods was also a part of the discrimination task (Belin et al., 1998a).

physiological studies (Werner et al., 2001; Ross and Pantev, 2004). The present study demonstrated activity in left posterior (BA39) and bilateral anterior (BA21) association cortices, when we analyzed the group mastering the temporal tasks, in contrast to the frontal and occipital activity seen in the group of poor performers (Table 3). Because a diVerence emerged between the groups, activity in the group of good performers is not a more general response to stimuli to the auditory association cortices, but more likely the result of a failure of analysis of degraded sensory information at the subcortical level. We found left asymmetry in gap detection. Earlier studies identiWed a right ear advantage and left hemisphere specialization of gap detection (Nicholls et al., 1999), which was also observed in gap detection involving other sensory modalities (Nicholls and Whelan, 1998; Stoesz et al., 2003). Although studies of normal hearing usually Wnd a correlation between gap-detection thresholds and word scores in competition with babble (Snell et al., 2002), tests of CI candidates do not conWrm that gap-detection thresholds predict speech scores (Waltzman et al., 1990; van Dijk et al., 1999). The present results highlight the cortical basis for these earlier Wndings by demonstrating that the TDL performance of CI candidates, in contrast to their performance in gap detection, depends on activity in the right posterior temporal lobe.

4.3. The role of cerebellum in timing

4.5. Deactivations

Activity rose in the left cerebellum in the subjects doing well on the TDL test. The role of cerebellum in encoding time intervals is controversial (Rao et al., 2001; Harrington et al., 2004; Ivry and Spencer, 2004). The Wnding of cerebellar activity in this group of postlingually deaf subjects, but not in normally hearing subjects under identical task conditions (Mortensen et al., 2005), supports the view that the cerebellar activity generally observed in sensorimotor and cognitive processing reXects monitoring and adjustment of input from the cerebral cortex rather than a speciWc timekeeping operation per se (Bower, 1997). This observation also implies that this part of the brain acts in support (possibly required for speech comprehension with a CI) of extraction of temporal cues from acoustic signals, as suggested by Ackermann et al. (1999).

Decreases of rCBF in frontal, parahippocampal and posterior regions including cingulate cortex, cuneus/precuneus and lingual gyrus may reXect processes active during a conscious, uncontrolled baseline condition (Shulman et al., 1997). Although frontal and occipital deactivations just below signiWcance did occur in the condition of passive listening to the electrical bursts, subtraction of the silent baseline failed to elicit any signiWcant decreases in rCBF, as expected according to the claim that unfocused conceptual processing may occur both during resting states and passive presentations when a focused perceptual task is not active (Binder et al., 1999).

4.4. Gap detection

This study shows that duration discrimination in postlingually deaf subjects depends on activity in speciWc regions of the cerebral cortex that include the right posterior temporal lobe in association with the right prefrontal cortex. The study conWrms previous results of normal hearing and may provide a cortical basis for why, in some previous studies, TDL was observed to be a predictor of postimplantation speech perception performance (Waltzman et al., 1990; van Dijk et al., 1999). The results also

Subjects performing better on the TDL task generally also performed better in gap detection (Fig. 2) and primarily engaged the same region (left posterior MTG) as subjects performing at levels at or below 150 ms (Table 3). In previous functional neuroimaging of normal hearing, we concluded that gap detection engaged processes below the cortical level, as indicated also by electro-

5. Conclusion

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M.V. Mortensen et al. / Hearing Research 209 (2005) 32–41

emphasize the potential role of training of time discriminatory ability in a hearing rehabilitation setting.

Acknowledgments Supported by The National Association of Hearing Impaired in Denmark, Desirée & Niels Ydes Foundation, and The Danish National Research Foundation’s Center of Functionally Integrative Neuroscience. The MR Center, Skejby Sygehus, generously provided some MR-images.

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